Feed-back circuits



Nov. 26, 1935. H. Av WHEELER 2,022,067 7 Y FE D BACK CIRCUITS Filed May 28, 1951 INVENTOR I #215010 A. WHEELf/f, Y v

ATTORNEYS [Patented Nov. 26, 1935 UNITED STATES "PATENT v OFFICE FEED-BACK CIRCUITS Harold A.

Wheeler, Great Neck, N. Y., assignor to Hazeltinc Corporation, a corporation of Delaware Application May 28, 1931, .Serial No. 540,581

6 Claims. (Cl. 250-20) This invention relates to wave regeneration, and has for its principal object the provision of a feed-back, or oscillator, circuit utilizing a vacuum tube or other space discharge device coupled with a resonant circuit which is tunable charge. device is made automatically dependent on the frequency of tuning, by the use of cou-' pling elements which remain physically fixed during tuning and which preferably do not rely on resistance for the performance of their functions. I I I I A particularly useful application of the invention is found in an oscillating or regenerative circuit which is tuned by a variable condenser in parallel with a fixed inductance. Two or more reactance elements may be used in the wellknown feed-back coupling circuit which is familiar in connection with the common triode vacuum tubes; and these elements may be chosen so that their individual contributions to the total feed-back coupling vary in different manners ;over the frequency range of'tuning. The several kinds of couplings can then be proportioned so that the total coupling is caused to vary in any predetermined manner with the frequency of tuning.

Another very useful application ofthe invention employs in the feed-back coupling system one inductance whose resonant frequency in the circuits is substantially above the tuning range and another inductance which is resonant slightly below the tuning range. Each inductance contributes a greater degree of coupling ,at frequencies near its resonant frequency than at frequencies more remote from resonance. The total amount of coupling can therefore be 'given any predetermined variation by properly proportioning the individual coupling components.

Another method of practicing the invention lies in utilizing combined capacitive and induc- I tive couplings in the feed-back circuit.

Improvements which are realized in the present invention are the following:

In accordance with the presentinvention, the amplitude of oscillation of a space discharge oscillator can be maintained substantially uniform while the oscillator is tuned over a wide frequency range, or on the other hand, can be made to vary with tuning in any predetermined disturbances.

manner. The desired amplitude vs. frequency relation is obtained by proportioning the different types of coupling to provide the necessary resultant coupling variation.

It is possible to constructan oscillator system in which the feed-back is so chosen that the amplitude of oscillation is uniformly low over the .entire tuning frequency range, as compared with the maximum output which can be obtained from the particular oscillator device employed. m This effect is obtained without adding resistance or damping effects in any appreciable amount to the tuned circuit of the oscillator system. Both the low amplitude of oscillation and the retained high selectivity of the tuned circuit 5 I contribute to the result that the output of the oscillator contains an extremely small percentage of harmonics or of distortion of the sine wave form. This result cannot be easily obtained by eitherof these features alone.

In the same manner it is possible to proportion the feed-backso that it is held just below the point of oscillation over the entire tuning range. This is a condition favorable to regenerative amplification, and has the advantages that the re- 25 generative feed-back does not have to be conduring tuning and that squeals heard during the tuning tinualiy adjusted and whistles are not process. I Y I The features f small harmonic distortion and of small amplitude of oscillation are especially advantageous in the beat oscillator of a heterodyne receiver, particularly in the familiar superheterodyne receiver. The feature of small harmonic distortion contributes to the prevention of audio beat notesbetween harmonics of the oscillator and undesired signals, thus greatly selectivity and the freedom from The feature of lowand uniform amplitude enables the functions of heat oscillator and modulator beat detector to be combined in a single tube of a heterodyne or a superheterodyne receiver without sacrificing perfor'mance in any way. I I I An oscillator with uniform output has advantages which are apparent if used in testing equipment where it is desired to avoid any unnecessary manipulation of the apparatus and, at the same time, to maintain a uniform output level. relied upon in the circuits of the present invention makes the frequency of the oscillator much more stable than it would be if resistance effects were utilized. This result is also realized when the invention is utilized in heterodyne receivers.

improving the The-fact that resistance effects are not my The application of this invention to regenerative amplification, described above, is quite similar to its application in an oscillating detector used for self-heterodyne reception. It is well known that this last function requires a low amplitude of oscillation; and the maintenance of a low amplitude without the use of auxiliary controls is a great advantage.

Of the drawing:

Fig. 1 is a circuit diagram of a radio receiver employing a vacuum tube detector-amplifier provided with feed-back circuits for regeneration or heterodyne reception in accordance with th present invention;

Fig. 2 is a circuit diagram of a vacuum tube oscillator system which is particularly applicable to radio testing equipment;

Fig. 3 is a circuit diagram of a vacuum tube oscillator system which can easily be designed to provide a uniform output or an output which va ries in any desired manner; and

Fig. 4 is a circuit diagram of an oscillator system utilizing a tetrode; or a so-called screen-grid vacuum tube, in addition to the features of this invention.

The use of the word vacuum-tube in this specification is not intended as a restriction of the invention to the ordinary high vacuum tubes.

9 The invention is equally applicable to many other types of, oscillating circuits. Particular reference is made to other types of oscillator devices employing a low pressure gaseous discharge, an electric arc discharge, or a secondary emission discharge such as the so-called dynatron" effect. The present invention will be described as applied to the triode va'cuum tube but is equally applicable to the other types of oscillating circuits.

The specific circuits described herein are intended to be tunable over the broadcast range of 550 to 1500 kilocycles per second or over the. slightly different frequency range required in the oscillator of a superheterodyne receiver (for example 725 to 1675 kilocycles per second). The

- 1 and 9.

invention is equally applicable to other frequency ranges utilized in so-called long wave receivers or short wave receivers.

Fig.- 1 illustrates a single tube radio receiver comprising an antenna I and a ground 2 coupled to a regenerative detector-amplifier tube 9 of the three-electrode type. The antenna i is coupled to the grid of tube 6 through a variable condenser 4, and the cathode SI of the tube is connected directly to ground. The system includes a resonant tuning circuit comprising the variable condenser 4, an inductance 5 connected directly between the grid and cathode of the tube and a fixed condenser 3 connected between the antenna and ground. The condenser 3 may be omitted if the antenna has sufliciently large capacity to ground.

The anode circuit of thetubet includes in series between the anode 62 and the cathode, in the order named, inductances I, 9 and 8,'headphones l8, and an anode battery H. A fixed condenser I of relatively low reactance to radiofrequency currents is connected between the low potential end of inductance 8 and the cathode for the purpose of'by-passing these radio-frequency currents past the battery I! and headphones IS. A condenser 10 is connected between the cathode and the point between inductances The inductances I and 8 are each inductively coupled to inductance 5, as indicated. by the battery may be replaced by any equivalent such as an alternating current supply equipped 'with rectifiers and filters.

Considering for the moment the regenerative 19 system, this comprises the two feed-back coils-' 1 and 8, each coupled to the inductance 5. Coil I has a relatively low inductance and is resonant together with inherent capacities at a frequency substantially higher than the tuning range of II variable condenser 4. On the other hand, coil 8 may have somewhat higher inductance and is included in series with the fixed inductance 9 and condensers l0 and II. This series circuitis resonant at a frequency slightly below the tun--fl ing range. This is accomplished either by making the total inductance of coils 8 and 9 large, or the capacities of condenser l0 and i l large, or both.

In obtaining the advantages of the present in 38 vention from the circuit of Fig. 1, the following method of operation is employed: the feed-back coil 1, it isnoted, has a relatively large feedback effect at the higher frequencies. This is the recognized behavior of the. well known tickler 80 coil. 0n the other hand, feed-back coil 8 with its associated inductance andcapacity has a relatively larger feed-back effect on lower frequencies approaching its own resonant frequency.

As a result, the amount of mutual inductance becoils 5 and 8.

pling will vary gradually over the frequency range at the rate determined by these two adjustments. 45 If desired, the low frequency adjustment can be made by varying the capacity of condenser I0, instead of by varying the mutualinductance of It is usually desired, in the system of Fig. 1, 60 that the two feed-back coupling components be combined in the proper polarities to aid each other. "Because of the coil '8 being included in the circuit having a natural frequency below the tuning range, the radio-frequency current 55 through coil 8 is opposite in polarity to the anode "current through coil 1. Therefore, coils I and 8 must be coupled 'to coil 5 in opposite directions.

The above method of adjustment of the two feed-back coupling components is equally appli- 60 cable whether it is desired that the circuit give regenerative amplification just below the point of oscillation, or oscillate at a uniformly low amplitude for heterodyne reception.

Fig. 2 shows an oscillator system which em- 65 bodies thepresent invention in order to obtain a uniform voltage output when .tuned over a large range of frequency. To enable the invention to be readily practiced, there are given numerical values for the elements, which have been found 7 satisfactory. These values are not intended to constitute limitations upon the invention.

The system comprises a three-electrode oscillator tube 29 which may be of the I'll-A type, and a resonant frequency-determining circuit in- 78 denser l4 and one of condensers cludinga fixed inductance cell It, a variabletuning condenser l4 and a number of fixed condensers l6 and II. The condensers l3 are adapted to be connected in parallel with condenser It and the condensers I! in is connected to the cathode 63 of tube 23, and an intermediate point of the coil is coupled to the grid 64 of the tube through a fixed grid condenser 26 (250 it). The series combination of con- (one of condensers It being in parallel with condenser I4), is connected across the entire inductance l3.

There is provided a feed-back arrangement which includes in a series circuit from the anode to the cathode of the oscillator tube, the following elements in the order named: fixed condensers 23 (0.1 f.) and 2| (250 #141.) ,andaninductance l9 which is inductively coupled to coil l3, as indicated by the brackets. An inductance 22 is connected between the intermediate point of coil l3 and the point between condensers 2| and 23. The anode potential is furnished by a battery 25 (90 volts) connected between the cathode and theanode through a choke coil 24, ('7 millihenries). 1 The grid of the tube is negatively biased by a biasing battery 28 (about 20 volts) which is connected between the cathode and the grid of the tube The output of the oscillator is taken from across an output coil 20 which is coupled to inductance l6 and in circuit, in pairs. Each pair of series and parallel condensers enables the same tuning condenser I4 to cover a different frequency range. The variable condenser preferably has a capacity range of 42 to 900 f. The following table gives the combinations of series and parallel condens ers required to cover the given frequency ranges:

In order to secure'a practically. linear frequency calibration for the scale of condenser M over these various ranges in frequency, the condenser plates are made semi-circular but the axis of rotation is located eccentrically so that the radius oi. the entering edge of the rotor plates is onehalf the radius of the trailing edge. This shape gives very nearly linear calibration over the intermediate frequency range (900 to 1100 kc.) and only slight curvature in the frequency calibration of the other ranges. capacities has the advantage that a single'half turn of the tuning condenser covers a frequency range ofonly one-fifth the broadcast range and therefore the scale divisions are relatively large and easy to readaccuratel'y. Each .of the five frequency ranges has an individually engraved scale on the condenser dial. The-dissipation in thecondensers and switching system is negligible, and the feed-back is not varied when switching condensers, so that the output suffers no abrupt change with frequency at the boundaries between adjacent frequency ranges. I

The feed-back coil l3 hasa relatively low in- '-on the otherhand, has a series with condenser 14 by switch l5, as required. One end of coil l8 quencies.

The switch I5 operates to connect condensers This combination of preciably affect the resonant frequency because 10 their combined inductance is much smaller than that of coil 22. At high frequencies the reactance of coil 22 is so large that almost the entire radio frequency plate current flows through condenser 2| and coil". r By'connecting the feed-back circuits so that coil I9 is coupled to coil l8, in a reverse direction, while coil 22 is-connected to atap on'coill8, the feed-back current through condenser 2| andcoil I3 has an effect which is augmented'by-go 7 the feed-back'current through coil 22' and the lower half of coil I3, the latter making a substantial contribution only at the lower fre- It is well known that an oscillator tuned by a variable condenser requires a greater g5 amount of feed-back at the lower frequencies. thanat the higher frequencies if the output is to be maintained'ata uniform level over; a fre-- quency range This result is accomplished with the circuit arrangement of Fig. 2 as described.

Thiscircuit. is utilized with excellent results as one element of a standard signal generator for testing radio receivers. The-special tuning arrangement contributes greatly to the ease of operation of this equipment.

The coil structure of Fig. 2 is'preferably' constructed as follows and located in a cylindrical copper can, 3.25." in diameter x 4.3" in'length':

The coil l8 comprises 63 turns of No. 22 B 8: S gaugewire spaced 22 turns per inch on a cy- 4o lindrical form 2" in diameter; The tap off is approximately at the center. Feed-back coil I3 comprises l2turns of No. B 8: S gauge wire spaced 16 turns per inch on a cylindrical form 2 in diameter.- Qutput coil 20 comprises 45 turns of wire on a cylindrical form 1%, in diameter. Coil I9 is wound over the upper half of coil l8 as indicated in the diagram, in order to prevent so-called parasitic oscillations which adapted to the requirements of a beat oscillator I in asuperheterodyne receiver although it is also suitable for the other purposes mentioned herein.- The system comprises a three-electrode oscillator tube 30, which may be of the 227 type, and the associated frequency-determining and so feed-back circuits, as in the case of the oscillator of Fig. 2. The anode circuit comprises the series connection from the anode 61 to the cathode 88,

of a fixed condenser 35, (.001 f.) an inductance 34 and the parallel combination of a condenser e5 32 and resistance 40, (10,000 ohms). The tunable circuit comprises an inductance 33, a variable condenser 3| and the fixed condenser 32, which is preferably, but not necessarily, larger than the maximum capacity of tuning condenser 3|. 1 For example, the tuning condenser may have a maximum capacity of 350 #111. and the conis resonant with 5 a end to the grid 66 ofthe oscillator through a condenser and grid-leak arrangement comprising condenser 38 (30 lmf.) and high resistance 39 (1 megohm). The variable condenser 3| is connected between the grid end of inductance 33 and the cathode, which is grounded.

The rectified current in the grid circuit of the'tube is carried by the resistances 39 and 40; in this manner, a grid bias voltage is established,

which automatically assumes the correct valueduring operation.

The anode current is supplied .from a battery 31 (180 volts) through a resistance 36 (20,000 ohms) which, at the same time, serves the purpose of a radio-frequency choke.

The feed-back circuit comprises the condenser 35, the coil 34 coupled to coil 33, and the condenser 32. The'function of condenser 35 is to separate the direct and radio'-frequency components of the anode current. The coil 34 provides a feed-back coupling whose effect is relatively greater at the higher frequencies of v the tuning range than at the lower frequencies. Condenser 32 provides an aiding feed-back coupling whose effect is greater at the lower thanat the higher frequencies of the tuning range because of its greater reactance at the lower frequencies.

- The two feed-back couplings of cofl 34 and condenser 32 are so proportioned that the total.

feed-back coupling has the correct value over the entire frequency range to maintain the grid voltage of the oscillator constant. It is equally possible .to proportion these couplings in a different manner and thereby secure any desired variation of the oscillating grid voltage over the tuning range. 1 v

The system of Fig.3 is one of the simplest possible arrangements of the present invention. When used in a superheterodyne radio receiver as a beat oscillator, the condenser 32 can be made to serve two functions at the same time. The

maining condenser is required to tune the beat oscillator, whose frequency calibration must be difierent by the amount'of the desired beat frequency. The difference in calibration is accomplished in some cases by the use of a condenser in the position of condenser 32. In accordance with the present invention, this condenser may be used for both this purpose and for the purpose of maintaining the oscillator voltage at a uniform level.

The oscillator system illustrated in 4 has some points in common with that'of Fig. 3, and

. also has certain distinguishing features. It has the same advantages as that of Fig. 3 in regard to its (use with superheterodyne receivers and has .additional advantages which will presently be pointed out. The circuit arrangement includes a vacuum tube 4| of the tetrode, onscreen-grid type, which may be of type 224, and a resonant frequency determining circuit, or oscillation cir.-

cult,- including an inductance 44, a fixed conden ser 43, (700 Md.) and a variable condenser 42, (maximum 350 at), in series with each other. junctionbetween the condensers 4.2 and 43 is grounded and the junction point between con- .ductance 44, as indicated by the brackets.

denser 43 and inductance 44 is connected to the anode, or plate, 69 of the tube through an inductance 45 in series with a fixed-condenser 50, (about 200 t). The cathode 10 of the tube is connected to ground through an inductance 46 l in series .with the parallel combination of a radiofrequency by-pass condenser 41, (about 500 1411.), and a resistance 48, which serves the purpose of furnishing a. bias voltage between the grid 53 and the cathode. gether, are termed the main coil, and the inductance 46 an auxiliary coil. The screen grid 1|. is grounded through a battery 49. Inductances 45 and 46 are each magnetically coupled to infeed-back path including condenser 50, coil 45,

coupled to coil 44, and condenser 43.

The operation of the circuit is as follows: Coil 45 has a mutual inductance with coil 44 which is about half the self-inductance of coil 44. At high frequencies, this inductive coupling is the main feed-back from the plate circuit into the tuned circuit. At low frequencies, the reactance of condenser 43 has a value as high as one-third the reactance of coil 44. Therefore, the total mutual reactance between the plate circuit and the tuned circuit varies between the limits of 50% and 83% of the reactance of coil 44. The mutual inductance between coils 44 and 45 is of such polarity that this coupling aids the coupling of condenser 43. The mutual inductance between coils 46 and 44 is about one-fourth the self-inductance of coil 44. This ratio does not vary with frequency because only one kind of coupling namely, inductive,

is used in the cathode lead. The polarity of coil 46 is such that the alternating potential of the 40, cathode is intermediate between that of the grid (mro) and that of the plate.

The coupling values of coils 45andv 46 and condenser 43, as illustrated in the above example,- are proportioned so that the feed-back effect varies 45. over the tuning range in the same manner as.

- the dissipation in the tuned circuit, namely; in-

creasing at lower frequencies so that the'oscillating current increases at lower frequencies more rapidly than" the resistance decreases.

'The cooperation of the two kinds of feed-back yields a uniform oscillating voltage across coil 44 and also across coil 46.

It is possible to utilize the circuit of Fig. 4 in a superheterodyne receiver, and to combine in it 56,

the functions of ,beat oscillator and modulator. A signal frequency tuned circuit, designated T, can be inserted if desired in the grid lead of grid 53 without materially affecting the oscillations. I Condenser (about 200 micromicrofarads) and coil 5| (about 5 millihenries) can likewise be resonated at the super-audible beat frequency without affecting the frequency of the oscillator. The beat frequency amplifier of the superheterodyne can then'be coupled to coil 5|. The remain- 35.

der of thesuperheterodyne receiver assumes any well-known arrangement. I

In the circuit of Fig. 4, it is found that the oscillator voltage is greatly reduced if the oscillator is tuned to an even multiple of the natural frequency of coil 5|. This is the result of resonant circulating currents in the coil which occur by virtue of its inductance and inherent capacity. Therefore the natural frequeny of coil 5| should be greater than. half the highest frequency in the The inductances 44 and 45, t0- l0- 20 above and below said range.

oscillator tuning range. This relation is not prel cise, but is very useful as a guiding rule.

While the invention has been described with particular reference to a number of specific arrangements, these are intended merely to illustrate the essential features and the wide field of usefulness of the invention. The circuits described and the circuit constants mentioned are not intended to indicate any restriction of the scope of the invention.

What is claimed is:

1. 'A vacuum tube feed-back arrangement comprising in combination a vacuum tube having an input electrode and an output electrode, a resonant circuit tunable over a frequency range and coupled to one of said electrodes, and a feed-back circuit comprising two parallel paths coupled in aiding phase between the other of said electrodes and to said tunable circuit, said parallel'paths being. resonant at fixed-frequencies respectively 2. A vacuum tube feed-back arrangement comprising in combination a vacuum tube having an input electrode and an output electrode, a resonant circuit tunable over a frequency range and coupled to one of said electrodes',and a feed-back circuit coupled to the other of said electrodes and to said tunable circuit, said feed -back circuit having two parallel paths external to said tube and connectedin aiding phase, one of said paths being resonant above and the other being resonant below said frequency range.

3. The method ofoperating an'oscillator system tunable over a frequency range, which comp'rises inductively feeding back in the same phase energy from the output to the input of the system by two parallel paths and causing the energy fed back through one of said paths to increase and f the-energy fed back through the other of said 40 paths to decrease while the tuning frequency is increased. f-

somewhat greaterthan the maximum value of 4.- The method of maintaining a uniformly high degree of regeneration in an amplifier provided with input and outputcircuits and'tunable over a frequency range, which comprises inductively feeding back in the same phase energy from the 5 output to the input circuit by at least two parallel paths and causing the energy fed back through one of said paths to increase and'the energy fed back through the other path to decrease while said amplifier is being tuned, whereby the overall feed-back energy may be maintained substan tially uniform over said range.

'5. An oscillator system tunable over a frequency range, comprising .a vacuum tube having a cathode, grid, and plate, a connection between 15 i said grid and ground, a main coil having one end coupled to said plate, a variable tuning condenser connected'between ground and the other end of said'main coil, aflxedcoupling condenser con nected between ground and an intermediate point 20.

on said coil, and an auxiliary coil coupled to said main coil and connected between ground ar 1 said cathode, said coupling condenser having a value said tuning condenser. I 6. An oscillator system tunable over a frequency range, comprising a vacuum tube having a cathode, grid and plate, a connection between said grid and ground, a main coil having one end coupled to said plate, a variable tuning condenser 30 connected between ground and the other end of said main coil, a fixed coupling condenser connected between ground and an intermediate point on said coil, and an auxiliary coil coupled to said main coil and connected between ground and said 35 cathode, said coupling condenser having a value somewhat greater than the mean value of said tunin condenser.

HAROLD A. 

